This invention relates to a class of linear motors and their associated control and drive electronics. Well known in the art are linear motor systems of the type known as Sawyer motors after the original inventor Bruce Sawyer. These systems consist of a flat strip or sheet of magnetic material, such as electrical grade steel, with periodic grooves cut or etched in the surface forming teeth, and an opposed mechanism with toothed electromagnetic elements. The steel strip or sheet is referred to as a platen, while the electromagnetic mechanism is referred to as a forcer. The combination of a forcer, a platen, and some bearing mechanism to provide a small air-gap between the two, constitutes a complete motor.
If the Sawyer motor operates in only one direction, it is a linear motor. The platen in this case is a strip with teeth cut in a direction orthogonal to the direction of motion. The forcer for a linear motor may contain only a single set of electromagnets, although multiple sets may be used to increase force. If the Sawyer motor operates in two directions it is a planar motor. The platen in this case is a sheet with teeth cut in two orthogonal directions forming a grid of intersecting grooves. The forcer for a planar motor must contain at least two sets of electromagnets, one for each axis of motion, although more electromagnets are typically used. In the following discussion the terms “Sawyer motor” or “motor” are used to refer to a complete motor combination, the terms “Sawyer motor forcer”, “Sawyer forcer” and “forcer” refer to just the electromagnetic mechanism, while the terms “Sawyer motor platen”, “Sawyer platen” and “platen” refer to just the toothed magnetic sheet or strip. Unless specifically noted, these terms apply to linear or planar Sawyer devices interchangeably.
The grooves formed in both the platen surface and the forcer electromagnets, are typically filled with a strong, stable epoxy, and both the platen surface and the toothed forcer surface are ground and lapped to provide an air-bearing quality surface. The forcer is provided with a flexible cable assembly, termed an umbilical, which contains the motor coil leads and a compressed air supply tube. Passages in the forcer mechanism convey the compressed air supplied by the tube to an arraignment of small orifices or air jets, releasing the air into a thin gap which forms between the forcer and the platen in opposition to the strong magnetic attraction between these two members. Planar Sawyer motors almost universally utilize air-bearings. Linear Sawyer motors may utilize air-bearings, ball or roller bearings, or recirculating ball slide units.
Since linear and planar Sawyer motor platens are totally passive, multiple Sawyer motor forcers may be operated on a single platen, provided they are controlled to avoid collisions. An industry standard tooth pitch of 0.040″ has developed, although both smaller and larger dimensions are encountered along with metric pitches. Platens and forcers are available independently from a number of vendors, and provided the tooth pitch is compatible, a forcer from one vendor will function correctly on a platen from any other vendor. While planar Sawyer motors typically incorporate an air-bearing within the forcer, this is not universal, an air-bearing may be incorporated in the platen, or a mechanical framework with air-bearings or rolling contact bearings may be used to mount the forcer in relationship to a platen. The latter approach is commonly used with linear Sawyer motors, the forcers and platens being sold without bearings, which are supplied as part of the specialized machines into which the Sawyer forcer and platen are incorporated. Thus Sawyer motor forcers, both linear and planar, are often sold independently of both platens and bearing supports, the latter objects being custom designed for particular applications, while the forcers are generally available in a small range of stock sizes offering varying force and load ratings. By analogy, rotary motors are available in so-called “open-frame” kits, which include a rotor and stator without bearings, the latter function being provided by the customer's machinery. The rotary analogy of buying only half a motor, either the stator or rotor, from one manufacturer for use with the complementary component purchased separately is lacking. In this respect, the commerce of Sawyer motors is clearly distinct from that of the more familiar rotary motors.
The invention is described in relationship to planar Sawyer motor forcers without regard to the platens. Although planar forcers are shown in all drawings and described in the detailed description and operation, the same principles may be applied to linear Sawyer motor forcers as well.
The arraignment of electromagnetic toothed elements in the planar Sawyer forcer usually consists of four independent but similar units each of which functions as a single axis linear motor referred to as a linear motor segment. Two linear motor segments typically provide motion in a first axis while being offset mechanically in the orthogonal direction, while the other two linear motor segments provide motion in the orthogonal axis while being offset mechanically in the first axis direction. Further, the mechanical placement of linear motor segments within the forcer mechanism is typically chosen to balance rotational torque about the forcer mechanism's center of mass.
The flexible umbilical connects the forcer to a remote controller that contains control processors and power amplifiers. The controller can drive the forcer in any arbitrary vector lying in the plane of the platen. A small rotation, typically +/−3 degrees, about a vector normal to the platen plane is also possible if the linear motor pairs of each axis are differentially controlled. The forcer and remote controller contain all the active elements of the system while the platen is completely passive. This allows multiple forcers to share a single platen provided they are controlled to prevent collisions.
U.S. Pat. No. 3,376,578 to Sawyer described various 3-phase variable reluctance planar motors. This patent disclosed the now classic forcer design using air-bearings with four independent linear motor segments arraigned for balanced rotational torque and balanced distribution of normal force. U.S. Pat. No. 3,457,482 to Sawyer subsequently disclosed 2-phase hybrid variable reluctance designs that incorporated permanent magnets. Next Bruce Sawyer disclosed several methods (U.S. Pat. No. 3,836,835 and U.S. Pat. No. 4,009,428) for controlling such motors with higher precision using continuously variable currents in both open-loop and closed-loop methods. An additional hybrid linear motor segment design was also disclosed wherein one permanent magnet and two coils formed a functional linear motor segment, the most compact design to date. These early patents defined the class of magnetic designs which have become known as Sawyer motors.
Nocito, et al. (U.S. Pat. No. 3,878,411) disclosed an improved linear motor segment design based on four of the previously disclosed single phase hybrid variable reluctance elements. This design spaced the hybrid variable reluctance elements in an optimal manner to form a linear motor segment capable of higher intrinsic positioning accuracy. This design is known as the 2/4-phase motor design since although four single-phase elements are used in each linear motor segment, the elements typically use coil windings coupled in a particular manner to allow 2-phase drive. Numerous variations of these early magnetic designs have been disclosed with alterations in geometry, materials, phase number, number and arraignment of linear motor segments, and construction methods. The most common systems continue to use four linear motor segments with each segment using a 2-phase or 3-phase hybrid variable reluctance design. Linear Sawyer motors typically use a single linear motor segment of the same basic designs used in planar Sawyer motors.
Most Sawyer motor systems are operated as open-loop stepper motors. In the 1990's a number of position sensor designs for planar Sawyer motors were disclosed in patents and technical publications. Miller at AT&T (U.S. Pat. No. 4,893,071) disclosed a capacitance-based sensor, Hollis, et al. at IBM (U.S. Pat. No. 5,434,504) disclosed an improved magnetic sensor, and Lampson at Yaskawa Electric America (U.S. Pat. No. 5,818,039) disclosed an optical sensor system. All of the above sensors are integrated into the forcer. Closed-loop systems using an external laser interferometer are available, but they are of limited use since they restrict the use of multiple motors on a platen, cannot deal adequately with motor rotation, and are very expensive. A number of additional patents and disclosures have been made for Sawyer motor position sensors, but the designs have not proven technically or economically viable. Linear Sawyer motors have long been operated with external linear position sensors such as are commonly available from a number of suppliers. There are typically optical or magnetic devices and have long been used on machine tools, a much larger commercial market than the linear motor market.
Electronic drive systems coupled to Sawyer forcers are microstepping drives, similar to conventional stepper motor drives. For pure variable reluctance designs, unipolar amplifiers are preferred. Hybrid variable reluctance designs use bipolar amplifiers. In the 1970's and 1980's bipolar linear amplifiers were preferred to obtain the best performance. These had very poor efficiency and were large and bulky. From the mid-1980's H-bridge pulse-width modulated (PWM) amplifiers became preferred for efficiency and cost benefits.
A disadvantage of Sawyer motors is the high total number of amplifiers required for two axes of motion. A minimum of eight H-bridge amplifiers with 16 motor phase leads is required for the standard 2-phase, four-segment forcer. Sawyer forcers are often used with platens having dimensions in the 1 to 2 meter size range, which results in umbilical lengths ranging up to 6 meters. Umbilical mass and orientation impart linear and rotational loads on the moving forcer, which degrades performance. Since the umbilical cannot be effectively shielded, the PWM amplifiers incorporate bulky, expensive and power inefficient output filters to reduce the radiated electromagnetic emissions. The limitations of current control systems and umbilical designs make it difficult to achieve safety certification.
Thus considerable time and expense has been devoted to developing controllers specifically tailored to the peculiar requirements of Sawyer motors while achieving high performance, reliability, safety and economic competitiveness. Only limited success has been achieved to date, consequently Sawyer motors have only a marginal market position.